In Multi User-Multiple Input and Multiple Output (MU-MIMO) transmissions (for both Broadcast Channel and Interference Channel), the major performance drop is caused by the imperfect Channel State Information at the Transmitter (CSIT). This is because in current standards the MU-MIMO transmission strategy has been designed under the assumption of perfect CSI knowledge at the transmitter but is actually used in scenarios where CSI is imperfectly known at the transmitter. Moreover, given the CSIT feedback mechanism in current standardizations, the accuracies of the CSIT vary across subbands and users depending on the availability of wideband Precoding Matrix Indicator (PMI) and user-specific PMI. An interesting work is to design new transmission blocks that cope with the imperfect (instantaneous) CSIT and making use of the varying CSIT qualities to benefit the performance.
The metric considered in this disclosure is the Degrees of Freedom (DoF). It can be interpreted as the number of interference-free streams transmitted to each receiver. Mathematically, it is given by
      d    =                  lim                  P          ->          ∞                    ⁢              R                              log            2                    ⁢          P                      ,where R is the rate and P stands for the Signal-to-Noise Ratio (SNR).
To investigate the impact of the imperfect CSIT on the sum DoF performance, the terminology, CSIT quality, is introduced. The CSIT quality is considered within the range of 0 to 1, representing unknown CSIT and perfect CSIT respectively. Moreover, the CSIT qualities are likely to vary across subbands and users. This setup can be interpreted as a practical deployment in line with Long Term Evolution (LTE) by connecting the CSIT quality with the availability of wideband PMI and subband PMI.
With the classical MU-MIMO transmission, the sum DoF performance is N2−N1+N1ā+N1b, where ā and b respectively stand for the average CSIT quality of receiver 1 and receiver 2 across all the subbands. If Single User-Multiple Input and Multiple Output (SU-MIMO) is performed, the maximal sum DoF performance will be N2 if the transmitter only sends messages intended for receiver 2. The key ingredient of the achievability relies on the interference cancellation using the past CSIT and channel output.
Moreover, the optimal strategy for the Multiple Input and Single Output (MISO) case despite the existence of imperfect CSIT, but DoF loss is incurred if it is reused in the MIMO case. The transmitted signal is made up of private messages and common messages. Intuitively, since two receivers have different antennas, the number of common messages should be limited by N1, otherwise, Rx1 is unable to decode them. This causes space resource wasted at Rx2 as it should have decoded N2 streams of common messages at a time.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.